A dense wavelength division multiplexing-passive optical network system (Dense WDM-PON, hereinafter, referred to as WDM-PON) is widely recognized as a next-generation ultimate optical network. One of the most important considerations in the WDM-PON technology is that an optical transmission module has to be independent on wavelength even if a plurality of optical wavelengths is used. The WDM-PON technology meeting this requirement is being widely studied all over the world. Among various methods, at present, there are two methods that have been developed to a commercial level: one of the methods is a wavelength locking WDM-PON and the other one is a wavelength reuse WDM-PON.
The wavelength locking method uses the phenomenon that when a seed light is injected into a special Fabry Perot laser diode (FP-LD) from outside, only the light of a wavelength injected into the FP-LD is amplified and the light of other wavelengths is suppressed. As the seed light source, a broad band light source (BLS) is used. In this method, two kinds of BLSs are installed in the central office. One of the two kinds of BLSs provides seed light to the FP-LD located at an optical line termination (OLT), and the other kind of BLS provides seed light to the FP-LD mounted at an optical network unit (ONU). The light transmitted from the BLSs is spectrum-sliced as it passes through an optical wavelength multiplexer (WDM MUX) included in the OLT and an optical wavelength multiplexer (WDM MUX) mounted at a remote node (RN), and the spectrum-sliced seed light is injected into the FP-LD.
The wavelength reuse method uses a reflective semiconductor optical amplifier (Reflective SOA; RSOA) as a light source for communications. An optical signal containing downstream data transmitted from the OLT is converted into a similar continuous wave (CW) light by removing downstream information in the RSOA at the ONU, and the converted light is demodulated into upstream data and transmitted to the OLT. Therefore, the demodulated optical signal transmitted to the ONU from the OLT serves seed light to the RSOA mounted at the ONU. Meanwhile, the RSOA mounted at the OLT, too, requires seed light, and, generally, external light is used as the seed light. Conventionally, a BLS is used as the external seed light source. That is, a broadband light transmitted from the BLS is spectrum-sliced as it passes through the WDM MUX mounted at the OLT, and the spectrum-sliced light is injected into the RSOA.
Meanwhile, in the conventional method for spectrum-slicing a BLS light by the WDM MUX located on a communications link and injecting it into the RSOA, there is no chose but to accept the loss of optical power in a spectrum slicing process, and hence there is a difficulty in reality in increasing the optical power of seed light to more than a predetermined level.